1. Patrick Stewart
Appeared in: X-Men film series, Logan, Doctor Strange in the Multiverse of Madness
This should come as no surprise. Patrick Stewart was already a veteran of the stage and screen (and a pop-culture icon due to his role as Captain Jean-Luc Picard in Star Trek: The Next Generation) by the time that he made his debut as Charles Xavier in 2000's X-Men, but it was in that role that he earned himself perhaps his biggest legion of fans. Yes, Professor X was popular, but Stewart's portrayal was legendary.
There's an aura that Stewart gives off in any role; it's regal, almost, and it makes him captivating to watch. He commands your presence, and he certainly does that in the X-Men film series. The star was effortlessly able to capture Charles' warmth as the benevolent teacher at his school, his reserved attitude to just about everything, and his calm, knowledgeable demeanor. Charles is an incredibly complex character and Stewart took us on a journey with him to help us see why. He is Charles in every way, and that ensured that we never once questioned him in the role.
Seeing him return to the role in Doctor Strange in the Multiverse of Madness was an awe-inspiring experience, especially as he had the iconic yellow chair from X-Men: The Animated Series. It was only fitting that he play that universe's Professor X, because Stewart is - and probably always will be - the definitive Charles Xavier.